A plasma CVD process means a process for forming a deposited film on a substrate by converting a specified substance into a plasma to form highly active radicals and bringing the radicals into contact with the substrate, and a plasma CVD apparatus means an apparatus used for carrying out the plasma CVD process.
Hitherto, such a plasma CVD apparatus has comprised a plasma CVD chamber constituted of a vacuum vessel provided with a raw material gas inlet port and an exhaust port, and a device for supplying electromagnetic waves or the like for supplying the energy by which the raw material gas introduced into the plasma CVD chamber is converted into a plasma.
The plasma CVD process is based on the high activity of the radicals mentioned above, and a desired deposited film is formed by appropriately selecting the density of the radicals, the temperature of the body to be treated, and the like. An important factor in the plasma CVD process is to efficiently form desired radicals to bring about the formation of a deposited film.
As a medium for supplying the energy for generating a plasma, electromagnetic waves of a high frequency of about 13.56 MHz have conventionally been used. However, it has recently been found that by the use of microwave of a frequency of about 2.45 GHz, it is possible to efficiently form a high-density plasma and to concurrently heat the body to be treated. And the public attention has been focused on a plasma CVD process employing microwave, and along with this, several apparatuses for the process have been proposed.
For instance, there have been proposed a method of forming a deposited film of amorphous silicon (hereinafter referred to as "A-Si") as a component for use in semiconductor devices, electrophotographic photosensitive materials, image inputting sensors, image pickup devices, photovoltaic devices and other electronic devices, optical devices, etc. by the plasma CVD process employing microwave (hereinafter referred to as "MW-PCVD process"), and an apparatus for carrying out the method.
FIG. 9 is a sectional schematic view illustrating an example of such known MW-PCVD apparatus.
In FIG. 9, numeral 1 denotes a rectangular waveguide, 2 a microwave inlet window, 3 a plasma generating chamber, 4 a film forming chamber, 5 and 10 each a gas supply pipe, 6 an exhaust port, 7 a body to be treated, 8 a support for the body to be treated, and 9 denotes a metal mesh.
As shown in FIG. 9, the apparatus comprises the chamber 3 for generating a plasma by use of microwave and the chamber 4 for forming a film by the plasma, the plasma generating chamber 3 and the film forming chamber 4 being separated by the metal mesh 9 so as to prevent the microwave and charged particles from penetrating into the film forming chamber 4. The plasma generating chamber 3 is constructed as a cavity resonator, and the microwave propagated through the rectangular waveguide 1 are introduced into the chamber 3 through the microwave inlet window 2 made of a dielectric material such as quartz (SiO.sub.2), alumina ceramic (Al.sub.2 O.sub.3) and Teflon. The film forming chamber 4, in which the body to be treated 7 is disposed, is provided with the gas supply pipe 5 and the exhaust port 6 for evacuating the plasma generating chamber 3 and the film forming chamber 4.
When the MW-PCVD apparatus of the above construction is operated, microwaves are introduced through the rectangular waveguide 1 into the plasma generating chamber 3, and an electric discharged gas introduced through the gas inlet port 10 is converted into a plasma by the electric field energy of the microwave to form a multiplicity of radicals. Only the radical capable of penetrating through the metal mesh 9 are introduced into the film forming chamber 4, to collide with the gas supplied through the gas supply pipe 5, whereby it is possible to form a deposited film on the body to be treated 7.
In the use of the conventional MW-PCVD apparatus of the above-mentioned construction, however, there is an unsolved problem that when the rectangular waveguide 1 and the plasma generating chamber 3 constituting the cavity resonator are fastened to each other, input impedance matching is not achieved and, therefore, a major portion of the electric field energy of the microwave is reflected, hindering effective utilization of the energy.
As a method of solving the problem, a method has been employed in which electromagnets are disposed around the cavity resonator to effect ECR (electron cyclotron resonance) (Refer to Japanese Patent Application Laid-Open No. 55-141729 (1980)). The method, however, requires a magnetic flux density of 875 gauss, leading to a considerably large and heavy apparatus. In addition, since the apparatus is generally designed to have a cavity resonator in vacuum, the refractive index of the plasma generated by electric discharge becomes less than 1, resulting in that the cavity resonator does no longer exist (Refer to "Electric Discharge Handbook" complied by The Institute of Electrical Engineers of Japan, Part 4, Chapter 2, P.298). Further, in the case where a static magnetic field is developed by electromagnets, the current is varied by heating the coiled wire material and, therefore, it takes a long period of time to stably establish the ECR condition (namely, a magnetic flux density of 875 gauss) by controlling the current variation. Besides, a deviation from the ECR condition during the period causes a lowering in the microwave absorption coefficient, and it is difficult to raise the efficiency in using the electric field energy before the condition is stabilized.